[mapguide-internals] Server rendering incomplete tiles - Defect or Feature??

[Traian Stanev]

Yes, 6.5 had a yellow warning triangle appear next to the layer in the layer list in case the layer failed. In the case of the rendering service, there was no way to indicate that because the return result is just a raw image...

Traian


-----Original Message-----
From: mapguide-internals-bounces at lists.osgeo.org [mailto:mapguide-internals-bounces at lists.osgeo.org] On Behalf Of Jason Birch
Sent: Monday, May 04, 2009 1:48 PM
To: 'mapguide-internals at lists.osgeo.org'
Subject: Re: [mapguide-internals] Server rendering incomplete tiles - Defect or Feature??

I understand the reason for this behaviour, but I don't agree with it even for live apps.  I'd personally rather serve up an error message or broken image than a misleading or inaccurate map.

This is similar to the case where a broken filter can cause an entire layer to be drawn. In these cases, MapGuide is trying too hard to serve a map and is in fact doing a disservice to the map provider.

I have quite a few cases where the city could get in real trouble if a layer isn't drawn or is drawn without the intended filter.  For instance, missing the stream setback layer on our zoning/ocp map, showing capital projects that aren't in our 5 year plan on the 5 year capital plan map, etc.

MapGuide 6.5 didn't behave this way.  Althought it displayed the layers it could, it also threw errors when there were data access problems. I think this may be a case where when returning a single image only one of these behaviours could be chosen, but I think the wrong one may have won...

Jason


----- Original Message -----
From: mapguide-internals-bounces at lists.osgeo.org <mapguide-internals-bounces at lists.osgeo.org>
To: MapGuide Internals Mail List <mapguide-internals at lists.osgeo.org>
Sent: Mon May 04 10:17:34 2009
Subject: RE: [mapguide-internals] Server rendering incomplete tiles - Defect	or	Feature??


It's classic MapGuide behavior to not take down the whole map because of a single layer -- a single layer that could be having intermittent issues with things like network connectivity to its data source for example. This is because most MapGuide applications have historically been live apps (and not offline tile generators).

Traian


-----Original Message-----
From: mapguide-internals-bounces at lists.osgeo.org [mailto:mapguide-internals-bounces at lists.osgeo.org] On Behalf Of UV
Sent: Sunday, May 03, 2009 10:31 PM
To: MapGuide Internals Mail List
Subject: [mapguide-internals] Server rendering incomplete tiles - Defect or Feature??

I studied the Stylize Layer code in MappingUtil.cpp.

The exception handler (line 760-790) at the end of the layer loop is catching exceptions and not doing anything about it (except logging) AFAIK this would explain why the server generates incomplete == wrong tiles without letting us know about it.
If I is misinterpeted the code please tell me why.

WHATS HAPPENING

Currently, mapping errors including the FDO access are replaced with a simple log message and the layer is omitted from the currently rendered tile. This gives all kinds of visual effects on the tile.
On a large map its virtually impossible to identify the wrong tiles. 
Just rerendering might require more than a day.

Currently I believe the server is giving a wrong system response by delivering such a faulty tile without note via the API.
This would be a major defect for this special class of use cases.
Furthermore, the relation between the log message and the offending tile is very difficult to establish.

If somebody thinks this is correct, please let me know why.

POSSIBLE RESOLUTION
Rethrowing the exception will cause the server to return an exception via the HTTP API causing the tile to fail also on the client side.
It will also stall the server eventually, very similar to the current situation in that the workerthread will cease to serve requests.
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